Noise-induced hearing loss is one of the most common occupational diseases. Thirty million US workers are exposed to potentially hazardous workplace noise. Noise-induced hearing loss is considered to be totally preventable, yet questions persist about the overall effectiveness of industrial hearing conservation programs and how they can be evaluated and improved on an ongoing basis. Some scientific barriers include: 1. Controversy about how best to use audiometry to evaluate a hearing conservation program. 2. Incomplete understanding of risk factors for susceptibility to hearing loss. 3. No validated, early indicators of noise-induced hearing loss in an individual. 4. No clear case definition of noise-induced hearing loss which can be applied to audiometric data sets. 5. Insufficient longitudinal databases of noise exposed workers which include adequate information about noise exposure over time. In this study, a large longitudinal data set of noise-exposed workers in an aluminum smelter and coal mine will be assessed and cleaned. A plant visit to review company records will increase the accuracy of existing exposure assessments. Through a nested case control and cohort approach, the data set will be analyzed with the aims of: Aim 1. Determining the true rate of noise-induced hearing loss at the study site. Aim 2. Assessing the relative importance of host and environmental risk factors for noise-induced hearing loss. Aim 3. Identifying and validating early indicators of noise-induced hearing loss. Aim 4. Developing and validating a case definition of noise- induced hearing loss which can be used in future epidemiologic research.